Abstract: In this paper, the principle for determination of` the loss coefficients in the pipe fittings with some heavy polyatomic gases such as Uranium Hexafluoride (Ufe) at pressures below atomospheric is introduced, An analysis of reasonableness and reliability of the test facility used for this purpose is briefly made,. Hundreds of tests for such three mediums as OF6C7F14, and N2 being conducted, the loss coefficients in Seventeen kinds of pipe fittings have been obtained, The loss coefficients in the various pipe fittings, the so-called correlation curves of the loss coefficients in pipe fittings and the friction coefficients in straight pipes versus Reynolds Numbers, and several empirical equations are given, All these can be used in engineering projects, The experimental results indicate that the compressible property of these gases must be taken into account in order to determine the loss coefficients accurately under present conditions, that the loss coefficient in the combination of a few pipe fittings are considered as the sum of that of each pipe fitting, and that there are differences in the magnitude of loss coefficients depending on mediums used. It is interesting to note that all these conclusions differ from those of previous authors at home and abroad.